This invention relates to a process for removing hydrogen sulfide from a gaseous stream such as coal gas, and more particularly to such a process which is effective over a mid-temperature range of approximately 300.degree.-400.degree. F.
Recent interest has been directed to integrated energy conversion plants which combine a coal gasification system, a cleanup system for generated coal gas, and a combustion gas turbine system for the production of power from the gas. In the cleanup system of such an integrated plant H.sub.2 S, NH.sub.3 and other undesirable compounds contained in untreated coal gas (e.g. particulates, tar vapor, and vaporized alkali) are typically removed in a low temperature cleanup processes such as that depicted schematically in FIG. 1 and described in U.S. Pat. No. 4,150,953 (Woodmansee) which is incorporated herein by reference. Untreated coal gas exhausted from an associated gasifier system enters the cleanup system at approximately 1200.degree. F. The majority of the alkali vapor, tar and particulates are removed in a quench/scrubber process in which coal gas is adiabatically saturated with water, dropping the gas temperature to approximately 330.degree. F. The gas exhausted from the quench/scrubber process is then cooled to approximately 180.degree. F. prior to H.sub.2 S removal since the H.sub.2 S removal processes typically employed in such systems require relatively low gas temperatures (i.e. less than 250.degree. F.). After H.sub.2 S removal the gas is subsequently reheated and resaturated prior to its use as a fuel in an associated gas turbine.
Unfortunately, this required cooling and subsequent reheating of the gas to achieve effective H.sub.2 S removal introduces a thermodynamic inefficiency to the integrated energy conversion plant. Additionally, such cleanup systems are complex and capital intensive. Indeed, the capital costs of such a gas cleanup system may exceed that of an associated gasification system. Again, much of the complexity is due to the temperature limitation of conventional H.sub.2 S scrubbing processes which require gas cooling with resultant condensate handling, heat recovery and wastewater cleanup sub-processes.
Accordingly, it is a major objective of the present invention to provide a simplified sulfur removal process useful in an integrated coal gasifier/gas turbine power plant which is effective over a mid-temperature range compatible with the temperature of gas exhausted from a cooperating quench/scrubber system (i.e. approximately 300.degree.-400.degree. F). Certain currently available processes for the removal of H.sub.2 S may be operable over this mid-temperature range, however these processes present various application drawbacks. For example, one such proposed system employs CuSO.sub.4 to precipitate hydrogen sulfide from geothermal steam at 350.degree. F. ("Removing H.sub.2 S from Geothermal Steam", G. E. Coury and M. Vorum, CEP September 1977, p.93). The use of copper sulfate is intended to enable achievement of very low hydrogen sulfide concentrations which are typically required in geothermal steam cleanup systems. However, copper sulfate is expensive and the regeneration of spent absorbent (CuS) is a relatively complex and difficult process. Thus, it is another object of the present invention to provide a mid-temperature H.sub.2 S removal process which includes a simple absorbent regeneration step.
Additionally, certain gasification systems produce coal gas which requires additional scrubbing to remove ammonia therefrom in order to meet nitrogen oxides emission standards. However, the ammonia scrubbing processes currently available require a system separate from the H.sub.2 S removal system, thereby increasing both the complexity and the cost of the cleanup process. Thus, it is desirable and it is an object of the present invention to provide a gas cleanup process in which ammonia can be scrubbed from a coal gas simultaneously with hydrogen sulfide removal so as to obviate the need for a separate ammonia absorption system.
Furthermore, it is desirable to recover certain byproducts of a gas cleanup process. Thus, (NH.sub.4).sub.2 SO.sub.4, which is useful as a fertilizer, is a desirable byproduct of a H.sub.2 S and NH.sub.3 removal process. Elemental sulfur is also a desirable gas cleanup system byproduct which is conventionally extracted in a separate process such as in a Claus process. Accordingly, it is yet another object of the present invention to provide a gas cleanup process which affords a simplified and inexpensive recovery of byproducts from such a process. Such a cleanup system would enable the elimitation of those process steps indicated by cross-hatching in FIG. 1, resulting in a significantly simplified cleanup process.
In addition to the foregoing, a system useful in coal gas cleanup should also selectively absorb hydrogen sulfide and ammonia from a gaseous stream while rejecting both carbon dioxide and water vapor which are typically contained therein. The retention of carbon dioxide and water in the fuel gas is important since they are effective expansion fluids when employed in an associated gas turbine and can increase the power output thereof. Additionally, absorbent material used in such a gas cleanup system is preferably inexpensive, non-corrosive and contains no alkali metal.